Nano-Scratch Response of Diamond-Like Carbon Film Deposited by Pulsed Direct Current Magnetron Sputtering

Introduction

Several attractive properties (Zehnder & Patscheider, 2000; Dean & Chalamala, 1999; Robertson, 2002; Neuville & Mathews, 1997; Donnet, 1996) of diamond-like carbon (DLC) films make them useful for number of applications including magnetic hard disc, microelectromechanical (MEMS) systems, biomedical applications (Grill, 1999; Jiang, Lu, Bogy, Bhatia, & Miyamoto, 1995; Roy, Kavasnica, Schalko, Eisenmenger-Sittner, Vorlaufer & Pauschitz, 2005), etc. In all these cases, thin coatings with superior wear resistance and low friction coefficients are desirable and hence DLC proves to be the most suitable particularly for ambient condition application. Although several techniques (Dall’Asén, Verdier, Huck, Halac, & Reinoso, 2006; Kohzaki, Higuchi, Noda, & Uccida, 1992; Bootkul, Supsermpol, Saenphinit, Aramwit, & Intarasiri, 2014; Pauschitz, Schalko, Koch, Eisenmenger-Sittner, Kavasnica, & Roy, 2003; Ronkainen, Koskinen, Anttila, Holmburg, & Hirvinen, 1992) are available for depositing DLC films, ECR-MPCVD is a promising technique where deposition takes place at low substrate temperatures (Sarakinos, Braun, Zilkens, Mraz, Schneider, Zoubos, & Patsalas, 2012) as it forms superior quality plasma. However, sputtering is a more widely used deposition method because of its advantages such as: low cost of operation, simplicity and control of the process and film homogeneity. The properties of the films deposited by sputtering, can be changed by adjusting the processing conditions such as sputtering and reactive gas flow, deposition pressure, sputtering power etc. By pulsing the sputtering voltage, structural and mechanical properties of DLC films can be improved further. The film can be made denser and the sp³ binding content can be increased by Ar bombardment (Robertson, 2002; Alami, Sarakinos, Uslu, & Wuttig, 2009). Pulsed DC sputtering also helps in suppressing the arcs (Grill, 1997) that can be used to reduce defect density in the deposited films when target material for sputtering is porous graphite.

Grill (1997) and Donnet (1998) reported the friction properties of DLC film. The friction coefficient of hydrogenated diamond-like carbon (a-C:H) depends on the relative humidity. The friction coefficient measured at low humidity is found to depend on the carbon precursor (Erdemir, Eryilmaz, & Fenske, 2000). The friction coefficient also varies with the C/H-ratio in the film. The highest friction coefficient is obtained for the film deposited with acetylene (C₂H₂) as precursor. On the other hand, CH₄ as a precursor tends to form polymeric a-C:H films that have limited wear resistance due to their soft nature (Chowdhury & Laugier, 2004).

The nanotribological properties of DLC films have been studied using a variety of methods. Several investigators have assessed the strength and adhesion of carbon films by means of nanoindentation (Hsiao, Bogy, & Bhatia, 1998; Fang & Chang, 2006; Longothetidis & Charitidies, 1999; Fan, Nose, Diao, & Yoshida, 2013) and nanoscratch testing (Li & Bhushan, 1998; Deng, Scarf, & Barnard, 1997; Mate, 1993) to measure nanohardness and scratch resistance respectively. Employing scanning probe microscopy friction and adhesion properties of a-C:H films were investigated by Mate (1993b) and Perry et al., (1995). With the help of atomic force microscopy (AFM), nanotribology of carbon-based films was reported by several investigators (Beake, Hassan, Rego, & Ahmed, 2000; Kavasnica, Schalko, Eisenmenger-Sittner, Bernardi, Vorlaufer, Pauschitz, & Roy, 2006; Bogus, Gebeshuber, Pauschitz, Roy, & Haubner, 2004; Tomala, Pauschitz, & Roy, 2013). Despite extensive work in this field, basic understanding and systematic study of the sputtering conditions on the tribological properties is far from being satisfactory.